The present invention relates to the processing of a substrate in the manufacture of integrated circuits. More particularly, the present invention relates to methods and apparatus for determining an endpoint for a plasma etching process in a plasma processing chamber during the processing of semiconductor substrates.
Substrates, e.g., wafers or glass panels, are widely used in the production of electronic devices. By depositing layers of selected materials and etching those layers in accordance with predefined patterns, integrated circuits and flat panel displays may be formed out of wafers and glass panels respectively. Although wafers, and in particular silicon wafers, are mentioned throughout this disclosure to simplify discussion, it should be borne in mind that the invention disclosed herein applies to any kind of substrate, including wafers made out of other materials (e.g., GaAs) as well as glass panels.
Silicon wafers are typically manufactured using plasma processing chambers to perform plasma etching processes during the manufacture of the silicon wafer. These plasma-enhanced etching processes are well known to those skilled in the art. An important aspect of the etching process is properly determining the point at which the plasma etching process is complete. This point at which the etching process is complete is referred to as the endpoint for the etching process. If the etching process proceeds for too long, over etching occurs which may damage the integrated circuit. On the other hand, stopping the etching process too early may result in an incomplete etch which may prevent the proper formation of features in the integrated circuit required to produce a good integrated circuit. As the size or critical dimension of integrated circuits is reduced below the sub micron level, the proper determination of the endpoint of the etching process becomes more and more difficult.
In the past, the endpoint for a particular etching process has been determined by test etching a number of test wafers in order to determine the length of time required for the etching process to etch through the desired layers of the wafer using a specific regime of pressure, flow rates, and flow ratios of etchants. Once this length of time was determined, it was used to determine the endpoint of the etching process. However, because of variations in the thickness of various layers from wafer to wafer and because of variations in the etching rate, this approach of using a predetermined length of time to control the etching process is not very reliable, especially in the case of wafers designed with small critical dimensions.
More recently, optical emission detectors and optical signal analysis have been used to determine the endpoint of certain etching processes. FIG. 1 illustrates a plasma processing system 100 including this type of an optical emission detecting arrangement. Plasma processing system 100 includes a plasma processing chamber 102. System 100 has a gas fine 104 connected to a shower head 106 in chamber 102 for releasing etchant gases into the chamber. A chuck 108 is used to support a silicon wafer 110 during the processing of the wafer. In order to monitor the wafer during the processing of the wafer, chamber 102 also includes a window 112. Window 112 allows light, indicated by arrow 114, that is produced by the reactions within chamber 102 to be detected by optical sensors outside of the chamber.
As illustrated in FIG. 1, system 100 includes an optical sensor 116 for measuring the intensity of light 114 emitted through window 112 during the etching process. Optical sensor 116 produces an electrical signal indicated by arrow 118 which represents the intensity of light 114. A computer 120 uses a detection algorithm and electrical signal 118 to determine the endpoint for the etching process. Computer 120 generates an endpoint signal indicated by arrow 122 which is sent to a chamber controller 124. Controller 124 uses endpoint signal 122 to stop the etching process and moves on to the next step in the processing of the wafer.
As is known in the art, system 100 also includes energy sources for striking a plasma within chamber 102 and an arrangement for exhausting the byproducts of the etching process. In the embodiment shown, RF sources 126 and 128 are used to respectively energize shower head 106 and chuck 108 in order to strike a plasma within chamber 102. Also, an exhaust port 130 is used to exhaust the byproducts of the etching process as the byproducts are produced. Typically, a turbo pump or other such device is used to draw any byproducts of the etching process from chamber 102.
During the etching process, etch species and reactants in the processing chamber emit light when their excited electrons change energy states. Each species produces a unique wavelength of light, and the intensity of each wavelength of light emitted from the plasma is related to the concentration of that species within the plasma. As a wafer is being etched, a reaction equilibrium is generally sustained within the plasma until the layer which is being etched starts to clear or be filly removed. At this point, the increase in the concentration of the etchant species and the decrease in the concentration of the reaction product species causes the light intensities associated with these species to increase or decrease. By measuring the light emission intensity change associated with the chemical species in the plasma, an endpoint for the etching process can be determined.
Two types of endpoint determination methods are currently in use. The first and most common is the threshold method of determining the endpoint. In this method, a sensor is used to detect the intensity of a certain wavelength of light which is produced by one of the reactants of the etching process. Generally, when the intensity of the wavelength of light crosses a predetermined threshold, the computer signals that the endpoint has been reached. In the second method, the shape of a curve representing the changes in the intensity of a particular wavelength of light which is produced by one of the reactants is used to determine the endpoint. In this method, the computer monitors the electrical signal provided by the optical sensor and compares the shape of the signal over time to a predetermined shape. Once a match is found, the computer signals the endpoint for the etching process
Optical emission detection arrangements such as the ones described above have several drawbacks which may make them costly and unreliable. In a first example of the drawbacks of this approach, the window in the chamber will typically become cloudy after a relatively short period of time. This is caused by deposits of polymers or other reaction products within the plasma on the inner surface of the window. Because the optical emission method uses the intensity of light emitted from the chamber to determine the endpoint of the process, a cloudy window can substantially reduce the intensity of the signal causing inaccurate readings. These inaccurate readings can cause the system to indicate the endpoint at the wrong time resulting in damaged and unusable wafers. In many cases, this clouding of the window will occur in as few as 2000 minutes of operation of the chamber. This means that the system must be shut down and cleaned regularly substantially reducing the throughput for the system and increasing the costs of producing the wafers.
As a second drawback, as the critical dimensions of the devices being produced on the wafer become smaller and smaller, the optical signals or light produced by the reactants becomes weaker and weaker. This makes it more and more difficult to discriminate between the background noise (e.g. light from other sources) and the light produced by the reactant which is being used to detect the endpoint. This may result in not detecting the proper endpoint and may significantly reduce the yield for a given etching process. Because of the very high cost of producing wafers with complex integrated circuits, this reduced yield can be very costly.
As another disadvantage, using either the threshold or the shape method of detecting the endpoint requires a large number of wafers to be tested in order to establish an acceptable range of conditions within which the endpoint may be determined. This is because there is always some variation from wafer to wafer or in the flow rates and reaction rates of the etching process. In order to deal with these variations, a large number of test wafers are etched in order to establish a statistically acceptable range of conditions within which the endpoint is to be identified. In many cases, hundreds to thousands of wafers may be run to establish the desired range. This can again significantly increase the costs of processing the desired wafers.
As another example of the disadvantages of the optical emission approach to detecting the endpoint of the etching process, variations from wafer to wafer and/or the use of a complicated process in which many reactants are involved may make it impossible to find an acceptable range of conditions that can be used to identify the endpoint of the process. If this is the case, the approach of using a predetermined length of time to control the etching must be used. However, as mentioned above, the use of this approach may cause a large number of bad wafers to be produced thereby increasing the costs of processing the wafers.
In view of the foregoing, there are desired improved methods and apparatus for more accurately determining the endpoint of a plasma-enhanced etch process.